Ok, heres the hypothetical situation. The U.S., after being mistreated by Europe and Asia, decides to destroy its enemies (the entire world). Before laughing too hard, lets look at the facts
(1) The U.S. has 3 methods of nuclear weapons delivery: Subs, land based missles, and air dropped bombs
(2)The U.S. has the best tanks and aircraft. The best Europe and Asia could hope to do would be to match the technology the U.S. has.
(3) The U.S. is isolated from the rest of the world. To the North and South are friendly, non-military countries. East and West are 2000 miles of ocean.
(4) Nuclear missle shield.
The only country that could launch nuclear missles against the United States would be Russia. The efficency of these weapons would be greatly reduced due to the missle shield and the fact that Russia is in bad economic shape.
Realistically, if 'collateral damage' wasnt an issue, the U.S. could roll through Europe with ease. Asia might be a bit more difficult due to terrain issues. But with the largest airforce in the world, troops, tanks, and nuclear weapons could be anywhere needed within a matter of hours.
As for the troops, if the U.S. annexed Mexico, it would have more than enough soldiers.